Many multi-residential buildings built during the 20th century include no suite-level temperature control. Building temperature control is achieved by varying the temperature of the hydronic heating medium leaving the central plant. Control of the heating medium temperature is achieved using an “outdoor reset curve”, which defines the relationship between outdoor temperature and the temperature of the heating medium. For example, during a winter day the heating system control might measure an outdoor temperature of −10° C., and then use the outdoor reset curve to determine a suitable heating medium setpoint temperature of 70° C. The heating system control would then modulate the boiler plant output to maintain the heating medium temperature at this setpoint.
The outdoor reset curve is typically a linear relationship based on subjective values assigned by the technician when commissioning the heating control systems. The settings are rarely reviewed, and there is typically no systematic process or feedback for adjusting the outdoor reset curve. As a result, conventional heating control systems can result in wasted energy, unnecessary costs and greenhouse gas emissions.